The invention relates to a method for determining a position of a movable element of a linear actuator for a motor vehicle, wherein the movable element is movable by means of a magnetic field of a coil of a linear actuator and the determination of the position of the movable element is based on a change of an impedance of the coil. The invention also relates to a linear actuator for a motor vehicle with an electric coil, a movable element which is movable by means of a magnetic field of the coil of the linear actuator and with a control unit which is configured to determine a position of the movable element based on a change of an impedance of the coil.
Generally, knowing a coil current which is supplied to a linear actuator, for example a holding magnet for moving and/for holding a movable element, for example an anchor of the holding magnet and a force of the linear actuator that is proportionate thereto, allows determining when the movable element reaches a predetermined position. However, in order to accomplish this under conditions in which the movability of the movable element is influenced by further factors, for example by a temperature-dependent viscosity of an oil in which the movable element moves, the force acting on the linear actuator and with this the coil current would have to be adjusted to the influences of the further factors. For example the coil current could be controlled as a function of the temperature. Another possibility is to determine the position of the movable element independent of the coil current supplied for the moving and/or holding the movable element. For this purpose for example additional sensors can be used. However there are also methods that measure an anchor position of a magnetic anchor of the actuator via a change of the impedance or inductivity of the coil. These methods are based either on detecting a time constant of an increase or decrease of the coil current or, if the coil current is controlled by means of pulse width modulation, measuring the inductivity via the amplitude of the coil current at the frequency of the pulse width modulation.